Electrical contact member



aircrasates 2.200.087 snsc'mrcat oou'rac'r mamas.

James M. Kelly, Trafiord, 1m, assignor a Westinghousc Electric 8;Manufacturing Company, East Pittsburgh, Pa, a corporation ofPennsylvania No Drawing.

application September is, 1938, Serial No. 229,8$5

5 Ciaims. (C1. MiG-165) of the contact members that when heat treated itwill combine with the refractory metal and 15 form a refractory carbide,thereby increasing the hardness of the contact member. Such refractorycarbides are undesirable in the contact member since they increase theresistivity of the metal and lower the melting point of the 20refractory with which the carbon combines.

Other alloying metals have been added in addition to the copper orsilver in making the contact member in an efiort-to enhance the physicalproperties of the resulting contact member, 25 but such additions havethe disadvantage that they affect the oxidation rate of the contactmembers, introduce impurities therein, or detrimentally sheet theconductivity without appreciably increasing the physical properties ofthe so contact member.

An object of this invention is to provide a contact member having highphysical properties, wear characteristics and good conductivity. Anotherobject of this invention is to provide as a composite contact memberformed-from reiractory metal powders and an alloy of silver and copper.j

A er object of this invention is to provide a ontact member formed fromrefractory 4 metal powders and an alloy which is susceptible to an agehardening heat treatment to give high physical properties, goodconductive and corrosion resistance properties tothe contact member.

g5 Other objects of this invention will become apparent from thefollowing description and the appended claims. 7

In practicing this invention; a contact memher is formed froma pluralityof metals which 5 have been found to cooperate to give certain desiredcharacteristics. A base body of refractory metal. selected from thegroup consisting of tungsten and molybdeum and which has a. relativelyhigh melting point so formedas to" have V interstices therein mawniciiof am: does not possess the desired characteristics is impregnated witha silver base alloy of lower melting point, as will be disclosed morefully hereinafter to give a resulting product having the desiredcharacteristics. t

In forming the metal base here employed, refractory metal powders ofpredetermined size, ranging from 150 to 250 mesh are mixed withsuflicient organic bond to form a plastic puttylike mass which ismoldable or which may be 10 rolled or cast to a desired shape. Anysuitable organic bond which can be removed by drying 'or sintering theshaped mass to leave a porous structure may be employed. It is desirableto employ a bond which can be substantially completely removed so as notto leave a carbon residue which may combine with the refractory metal inthe form of a carbide, since such carbides are undesirable. An organicbond com posed of about 75% of shellac and 25% of alcohol is entirelysatisfactory in forming the putty-like mass, and when the mass is heatedat a temperature of about 1000 C. in a reducing atmosphere, issubstantially completely removed leaving the desired porous base ofrefractory metal.

In order to impart the desired electrical characteristics to the contactmember, the porous refractory metal base or body is then filled with'the impregnating silver base alloy. The alloy employed for impartingthe desired characteristics is composed of from 90% to 98% silver with rthe balance copper, a preferred alloy being composed of about 92.5% ofsilver and about 7.5%

of copper. I This alloy is susceptible to precipitation hardening andwhen heat treated, as by quenching it from a temperature of between 600C. and 775 C. and aged at a temperature of between 175 C. -and 300 C.has a high conductivity of 80%, or greater, and a hardness ap- 4,0proximating 90 Rockwell B, or greater.

Although the silver-base alloy disclosed herein has a melting point ofabout 800 C., it has been found necessary to subject the alloy and theporous refractory metal body to a temperature of between 1100C. and 1125C. in order to eflect the impregnation of the porous base. It is thoughtthat this is because the surface tension of the alloy is too great topermit successful wetting of therefractory metal at lower temperatures.At these temperatures, the alloy melts, wets and completely impregnatesthe porous refractory metal base, giving a very uniform structure inwhich the refractorymetal is securely bonded by the silver base alloy toproduce a strong homogeneous article.

For general applications, contact members of from to 75% by weight ofthe refractory metal powders impregnated with from to. 25% by weight ofthe silver base alloy having a copper content of from 2% to 10%by-weight are satisfactory. The silver base alloy having a coppercontent of 7.5% by weight is particularly eflective as the impregnatingalloy giving a contact member which is resistant to corrosion and whichhas high physical properties, that .is, a high hardness, high tensilestrength and resistance to wear or deformation and good conductivity.

In a contact member formed from 50% of tuna-- sten powders of 200 meshsize and impregnated, as described hereinbefore, with the. silver base.alloy having a copper content of 7.5%, the contact member is found tohave a hardness of 76 Rockwell B, as quenched in water from 750 C. Insuch annealed condition, the contact member having a tungsten content ofabout 50%, a silver content of about 46.2% and a copper content of about3.8% can be readily machined to any predetermined shape. Whensubjectedto a further treatment consisting of aging'the contact memherfor 18 hours at a temperature of 200 C., the

contact member is found to have a hardness 92 Rockwell B or greater.

By practicing this invention, contact members are produced having anextremely high hardness and good conductivity which are 'free from thecarbon contamination sometimes employed heretofore for improving thewearing characteristics of contact members. With the combination ofmetals employed, a contact member is produced Whichmay be machined toshape and which afterwards can be so hardened as to substantiallyprevent deformation of the contact member when subjected to repeatedblows, such as are' state of sub-division. Such contact members whetherin the form of contactor tips, electrodes,

or other forms, are resistant to corrosion and are successful in servicewhere it is necessary to interrupt heavy currents without danger ofcontact failure.

Although this invention has been described with reference to aparticular embodiment thereof, it

2. An electrical contact member comprisin from 45% to by weight ofrefractory metal powders substantially free from carbides and formed toa predetermined shape and impregnated with asilver-copper alloyofuniform composition having a silver content of from to 97.5%. byweightand which has been precipitationhardened subsequent to the impregnationto give high physical. strength and good conductivity. f

3. An electrical contact member comprising about 50%v by weight oftungstensubstantially free from carbides, about 46.2% by weight ofsilver and about 3.8% by weight of copper, the silver and copper beingpresent in the form of an alloy of uniform composition capable of beingprecipitation hardened. 4. An electrical contact member comprising about45% to 75% by weight of tungsten substantially free from carbidesimpregnated with from 55% to 25% by weight of a silver copperalloy ofuniform composition having a silver content of from 90% to 97.5% byweight, formed to a predetermined shape and precipitation hardened togive high "physical strength and high conductivity, 1

5. An electrical contact member as in claim 1 which has been quenchedfroma temperature of between 600 C. to 775 C. and aged at a temperatureof'between C. and 300 C.

JAMES M. KELLY.

